Method for removing metal ions from water with a nanocomposite film

ABSTRACT

A method of removing a metal ion from water is disclosed. The method includes treating the water with a nanocomposite to absorb the metal ion with the nanocomposite, forming a polymer-metal ion composite and removing the polymer-metal ion composite from the water. The nanocomposite includes aluminum oxide dispersed in a matrix of an uncrosslinked graft copolymer that includes a chitosan backbone and side chains of poly(itaconic acid) grafted to the chitosan backbone. The chitosan backbone has a plurality of amino groups that are acetylated by itaconic acid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to a U.S. patent application titled “IMIDAZOPYRAZOLE DERIVATIVES AND PREPARATION THEREOF” with Attorney Docket No.536936US.

BACKGROUND OF THE INVENTION Technical Field

The present disclosure is directed to methods of water treatment usingnanocomposites. More specifically, the present disclosure relates to amethod of water treatment using chitosan-graft-itaconic acid-aluminumoxide nanocomposites.

Description of Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Fresh drinking water is essential for human survival as the human bodyuses water for performing various vital functions. Water is needed byevery cell of the human body as the water helps to transport nutrientsand oxygen. The availability of fresh water for drinking from naturalresources is determined by rainfall. Water scarcity is becoming a globalproblem and is expected to worsen further in the coming decades.Additionally, the disproportionate increase in urbanization placesincreasing pressure on already stressed natural water resources. Yet,another impending and related concern besetting the human population iswater pollution. Industrial effluents and untreated domestic waste moreoften find their way to freshwater resources. On the other hand, organicand inorganic pollutants originating from either human activities orsoil leaching contribute towards polluting ground water. Industrialcontaminants from oil refineries, underground storage tanks, andtransmission pipelines often find their way into ground water. Due tothe persistent nature of these pollutants, the pollutants pose severalhealth risks to humans, animals, and other living organisms.

The oceans make up 70 percent of the earth’s surface and account for 96percent of the water on the planet. However, ocean water is not suitablefor human consumption as ocean water is saturated with salts. In recentyears, water purification methods have garnered a lot of attention fromresearchers and innovators. Nanoscale composite materials of highsurface area, chemical reactivity, mechanical strength, andcost-effectiveness show a huge potential for water treatment andpurification. Nanocomposites can eliminate bacteria, viruses, inorganicpollutants, and/or organic pollutants from wastewater in a variety offorms, such as chelation, absorption, ion exchange, or any othersuitable mechanism.

Several efforts have been made to utlize nanocompsites for solving thewater scarcity problem. However, drawbacks such as toxicity, adverseenvironmental effects, and expensive nature of the proposed solutionsmake them inappropriate and cumbersome to adopt.

US10913021B2 discloses a water purification device that includes a heavymetal removal layer, a biological species removal layer and a supportlayer. While the water purification device may also include chitosannanoparticles and/or aluminum oxide hydroxide nano-whiskers in separatelayers, the chitosan nanoparticles and/or aluminum oxide hydroxidenano-whiskers are not included in the heavy metal removal layer, butincluded in the biological species removal layer or an additionalanti-fouling layer.

Thus, there is an unmet need to provide suitable, simple, cost-effectiveand green solutions to combat water pollution and purify water for humanconsumption. Accordingly, it is one object of the present disclosure toprovide methods that utilize nanocomposites for water treatment andwater desalination.

SUMMARY

In an exemplary embodiment, a method of removing a metal ion from water,includes treating the water with a nanocomposite to absorb the metal ionwith the nanocomposite and form a polymer-metal ion composite andremoving the polymer-metal ion composite from the water. Thenanocomposite includes aluminum oxide dispersed in a matrix of anuncrosslinked graft copolymer that includes a chitosan backbone and sidechains of poly(itaconic acid) grafted to the chitosan backbone having aplurality of amino groups that are acetylated by itaconic acid.

In another exemplary embodiment, the method includes acidifying thepolymer-metal ion composite to remove the metal ion from thepolymer-metal ion composite and regenerate the nanocomposite. In anembodiment, the metal ion is Cu²⁺ ion and the acidifying includes mixingthe polymer-metal ion composite with nitric acid to remove the Cu²⁺ ionfrom the polymer-metal ion composite. In another exemplary embodiment,the method includes treating the water with the nanocomposite formed bythe acidifying to remove the metal ion from the water.

In another exemplary embodiment, the method further includes forming theuncrosslinked graft copolymer by adding sodium bisulfite, potassiumpersulphate and itaconic acid to chitosan in acetic acid to form asolution; heating the solution to an elevated temperature for a periodof time to form uncrosslinked graft copolymer; filtering the solution toobtain a crude product including the uncrosslinked graft copolymer andunreacted chitosan; and removing the unreacted chitosan from the crudeproduct by Soxhlet extraction with ethanol to obtain the uncrosslinkedgraft copolymer. In an example, the sodium bisulfite has a concentrationof 0.01-0.03 M in the solution; the potassium persulphate has aconcentration of 0.01-0.03 M in the solution; the chitosan has aconcentration around 0.1 M in the solution; the itaconic acid has aconcentration of 0.1-0.2 M in the solution; the elevated temperature isin the range of 30-60° C.; and the period of time is between 1 hour and6 hours.

In another exemplary embodiment, the method includes forming thenanocomposite by solution casting a mixture of the uncrosslinked graftcopolymer and aluminum oxide nanoparticles. In an embodiment, the methodfurther comprises dissolving the uncrosslinked graft copolymer in aceticacid to obtain a polymer solution and adjusting the pH of the polymersolution to the range of 6-7. A suspension of aluminum oxidenanoparticles is added in water portion-wise to the polymer solution toform the mixture followed by stirring the mixture with casting themixture onto a carrier substrate; and drying the cast mixture to formthe nanocomposite film. In an example, the aluminum oxide nanoparticleshave an average particle size of less than 50 nm.

In an exemplary embodiment, the treating of water includes immersing thenanocomposite in the water and shaking the water at an elevatedtemperature. In an example, the method further includes shaking thewater between 30° C. and 60° C. at a speed of 100-300 rpm for 1-3 hours.In an exemplary embodiment the method includes removing metal ion atleast one of Cu²⁺ and Zn²⁺. In another example, the metal ion includesat least one of Na⁺ and K⁺.

In an exemplary embodiment, the uncrosslinked graft copolymer includesthe side chains of poly(itaconic acid) grafted to the chitosan backbonevia C6 hydroxyl groups. In another example, a grafting density ofitaconic acid is between 20 wt. % and 60 wt. %, the grafting densityincluding poly(itaconic acid) grafted to the chitosan backbone and theitaconic acid acetylating the plurality of amino groups of the chitosanbackbone.

The foregoing general description of the illustrative embodiments andthe following detailed description thereof are merely exemplary aspectsof the teachings of this disclosure, and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this disclosure and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a graphical representation depicting effect of grafting timeon the grafting percentage of a chitosan-graft-itaconic Acid (CS-g-IA)copolymer, in accordance with exemplary embodiments of the presentdisclosure,

FIG. 2 is a graphical representation depicting effect of temperature onthe grafting percentage of a CS-g-IA copolymer, in accordance withexemplary embodiments of the present disclosure,

FIG. 3 is a graphical representation depicting effect of monomer (IA)concentration on the grafting percentage of a CS-g-IA copolymer, inaccordance with exemplary embodiments of the present disclosure,

FIG. 4 is a graphical representation depicting effect of initiatorconcentration on the grafting percentage of a CS-g-IA copolymer, inaccordance with exemplary embodiments of the present disclosure,

FIG. 5 is a graphical representation depicting carbon-13 nuclearmagnetic resonance (¹³C-NMR) spectrum of a CS-g-IA copolymer, inaccordance with exemplary embodiments of the present disclosure,

FIG. 6 is a graphical representation depicting ¹³C-NMR spectrum ofchitosan (CS), in accordance with exemplary embodiments of the presentdisclosure,

FIGS. 7A and 7B illustrate morphology of chitosan and a CS-g-IA/ Al₂O₃nanocomposite, respectively, in accordance with exemplary embodiments ofthe present disclosure,

FIG. 8 is a graphical representation depicting thermal gravimetricanalysis (TGA) of various grafting percentage of CS-g-IA copolymers ascompared to CS, in accordance with exemplary embodiments of the presentdisclosure,

FIG. 9 is a graphical representation depicting effect of pH onadsorption of Cu (II) ions by a CS-g-IA copolymer (G% = 52.65%) ascompared to CS, in accordance with exemplary embodiments of the presentdisclosure,

FIG. 10 is a graphical representation depicting effect of pH onadsorption of Zn (II) ions by a CS-g-IA nanocomposite (G% = 52.65%) ascompared to CS, in accordance with exemplary embodiments of the presentdisclosure,

FIG. 11A is a graphical representation depicting effect of temperatureon adsorption of Cu (II) ions by a CS-g-IA nanocomposite (G% = 52.65%)as compared to CS, in accordance with exemplary embodiments of thepresent disclosure,

FIG. 11B is a graphical representation depicting effect of temperatureon adsorption of Zn (II) ions by a CS-g-IA nanocomposite (G% = 52.65%)as compared to CS, in accordance with exemplary embodiments of thepresent disclosure,

FIG. 12 is a graphical representation depicting X-ray diffractionanalysis (XRD) of a CS-g-IA copolymer (G% = 52.65%): (a) before removalof Cu (II) ions (b) after the removal of the Cu (II) ions, in accordancewith exemplary embodiments of the present disclosure,

FIG. 13A is a graphical representation depicting energy dispersive X-rayspectroscopy (EDS) of Cu (ions): after treatment with a CS-g-IAcopolymer, in accordance with exemplary embodiments of the presentdisclosure,

FIG. 13B is a graphical representation depicting Energy dispersive X-rayspectroscopy (EDS) of Cu (ions): after a regeneration process, inaccordance with exemplary embodiments of the present disclosure,

FIG. 14 is a graphical representation depicting recovery percentage as afunction of the volume of 0.5 mol/L HNO₃, in accordance with exemplaryembodiments of the present disclosure,

FIG. 15 illustrates a proposed mechanism of NaCl removal by salt bondingto a CS-g-IA nanocomposite according to an exemplary embodiment, inaccordance with exemplary embodiments of the present disclosure,

FIG. 16A illustrates morphology of CS-g-IA nanocomposite after removalof NaCl by characterizing the Na cations, and

FIG. 16B illustrates morphology of CS-g-IA nanocomposite after removalof NaCl by characterizing the C1 anions.

DETAILED DESCRIPTION

In the drawings, like reference numerals designate identical orcorresponding parts throughout the several views. Further, as usedherein, the words “a,” “an” and the like generally carry a meaning of“one or more,” unless stated otherwise.

Furthermore, the terms “approximately,” “approximate,” “about,” andsimilar terms generally refer to ranges that include the identifiedvalue within a margin of 20%, 10%, or preferably 5%, and any valuestherebetween.

The term, “nanocomposite”, as used herein, refers to multiphase materialwhere one of the phases has one, two or three dimensions of less than100 nanometers (nm) or a material resulting due to amalgamation ofmatrix continuous phase matrix and a nano-dimensional materialdiscontinuous phase.

The term “alkyl”, unless otherwise specified, refers to both branchedand straight chain saturated aliphatic primary, secondary, and/ortertiary hydrocarbons of typically C1 to C21, for example C1, C2, C3,C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, and specificallyincludes, but is not limited to, methyl, ethyl, propyl, isopropyl,cyclopropyl, butyl, isobutyl, t-butyl, pentyl, cyclopentyl, isopentyl,neopentyl, hexyl, isohexyl, cyclohexyl, cyclohexylmethyl,3-methylpentyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 2-ethylhexyl,heptyl, octyl, nonyl, 3,7-dimethyloctyl, decyl, undecyl, dodecyl,tridecyl, 2-propylheptyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl,octadecyl, nonadecyl, and eicosyl.

The term “chitosan”, as used herein, comprises high molecular weightchitosan and low molecular weight chitosan, wherein the chitosan can beviewed as chitin with a degree of deacetylation that is typically atleast about 70%, at least about 75%, at least about 80%, at least about85%, at least about 90%, at least about 95%, at least about 99%, or thechitin can be substantially fully deacetylated.

The term “Al₂O₃” or “aluminium oxide”, as used herein, comprisesaluminium oxide nano powder.

The term “graft copolymers”, as used herein, refer to copolymers with amain polymer chain, also known as the backbone, having one or more sidepolymer chains attached to it through covalent bonds.

The term “wastewater”, as used herein, refers to any form of used waterand includes water that has either been affected by domestic,industrial, and commercial use or water that is used to transport waste.

The term “desalination”, as used herein, refers to any process or methodor a device used to remove salts and minerals from a liquid includingwater irrespective of the source of the water.

Aspects of this disclosure are directed to a method of removing a metalion from water. The method includes treating the water with ananocomposite to absorb the metal ion with the nanocomposite and form apolymer-metal ion composite. The nanocomposite includes aluminum oxidedispersed in a matrix of an uncrosslinked graft copolymer that includesa chitosan backbone and side chains of poly(itaconic acid) grafted tothe chitosan backbone having a plurality of amino groups that areacetylated by itaconic acid. The method further includes removing thepolymer-metal ion composite from the water. In some embodiments, theside chains are distributed randomly along the backbone. In someexamples, the side chains are grafted by “grafting to,” “grafting from,”or “grafting through” or a macromonomer method.

In an exemplary embodiment, the method includes acidifying thepolymer-metal ion composite to remove the metal ion from thepolymer-metal ion composite and regenerate the nanocomposite. In someembodiments, the acidifying includes use of an acid belonging to a groupcomprising nitric acid, sulfuric acid, hydrochloric acid, phosphoricacid, citric acid, hexanoic acid or a combination thereof. In someembodiments, the metal ion removed by the method consists of a groupincluding copper, zinc, mercury, cadmium, chromium, lead, gold, uranium,arsenic, sodium, and potassium and alloy or oxide or a mixture thereof.In an exemplary embodiment, the acidifying includes mixing thepolymer-metal ion composite with nitric acid to remove the Cu²⁺ ion fromthe polymer-metal ion composite. In some embodiments, the nitric acidused for the acidification is present as an azeotrope with anotherliquid at a concentration of up to 70%.

In another exemplary embodiment, the method includes treating water withthe nanocomposite formed by the acidification to remove the metal ionsfrom the water. In some embodiments, the nanocomposite consists of agroup including membranes, films, sheets, coatings, and particles withimproved adsorption capacity. In some examples, the nanocomposites arepart of a portable device for water treatment for industrial, domestic,or scientific applications.

In an exemplary embodiment, the method includes treating the water withthe nanocomposite membrane comprising aluminum oxide dispersed in amatrix of the uncrosslinked graft copolymer disposed within the polymermatrix and the film is substantially permeable to water andsubstantially impermeable to impurities. In some examples, theimpurities include at least one of a sodium ion, a potassium ion, amagnesium ion, a calcium ion, a silicate, an organic acid, or anonionized dissolved solid with a molecular weight of greater than about200 Daltons or a mixture thereof.

In an exemplary embodiment, at least about 50, 55, 60, 65, 70, 75, 80,85, or 90% of the aluminum oxide nanoparticles can be positioned betweenvarious surfaces of the film. In one embodiment, the aluminum oxidenanoparticles can be substantially encapsulated within the film. Theterm “encapsulated,” means that at least about 70, 75, 80, 85, or 90%the nanoparticles can be positioned between the surfaces of the film.

In an exemplary embodiment, the nanocomposites of the method may furtherinclude some additional nanoparticle materials such as, SiO₂, TiO₂, andthe like. Other materials suitable for use as nanoparticles includemetals, such as, silver (Ag) for antimicrobial activity. In someexamples, additional nanoparticles known for high thermal conductivityare also included in the nanocomposites, such as, bromate (BrO₃).

In an exemplary embodiment, the treatment of the water with thenanocomposite formed by the acidifying to remove the metal ions from thewater includes reverse osmosis. In another exemplary embodiment, thewater treatment method is forward osmosis. In an example, thenanocomposite is used for filtration or ultrafiltration of water. Inanother example, the nanocomposite is used for the treatment ofwastewater in a wastewater treatment plant. In yet another example, thenanocomposite is used for the treatment of saline water in adesalination plant.

In an exemplary embodiment, the method of the present disclosure isapplied to an apparatus for the desalination of water having afunnel-and-gate configuration wherein the gate comprises one or morelayers of the graft copolymer nanocomposite membrane or film for theremoval of impurities from the water. In an example, the impuritiesinclude metals such as copper, zinc, sodium, and potassium and alloy oroxide or a mixture thereof. The graft copolymer nanocomposite may removeone or more metals at the same time with equal specificity for themetals present in the water. In some examples, the metal ions includeCu²⁺ ions and the method further includes mixing the polymer-metal ioncomposite with nitric acid to remove the Cu²⁺ ions from thepolymer-metal ion composite. In another example, the metal ions includeat least one of Zn²⁺, or Na⁺ or K⁺.

In an exemplary embodiment, the branched and uncrosslinked graftcopolymers exhibit adsorption efficiency such that the Cu²⁺ and Zn²⁺ areremoved from the wastewater contaminated with salts including CuC1₂ andZnC1₂. In yet another exemplary embodiment, the branched anduncrosslinked graft copolymers show desalination efficiency for theremoval of cations such as Na⁺ or K⁺ from saline water. In someexamples, the method includes regeneration of the cations outside thecopolymer matrix using acidification. In some embodiments, thepercentage of regeneration is at least between 50 and 60, 60 and 65, 65and 75, 75 and 80, 80 and 85, 85 and 90, 90 and 95, 95 and 100% of themetal ions present in a sample.

The method also includes using the graft copolymer exhibiting propertiesof reusability for multiple times. In some embodiments, the methodsinclude using the graft copolymer for at least 4 cycles. In someexamples, the method includes using the graft copolymer wherein themetal ion removal efficiency remains almost unaltered after severalcycles of the metal scavenging process.

In another exemplary embodiment, the method further includes forming theuncrosslinked graft copolymer by adding sodium bisulfite, potassiumpersulfate and itaconic acid to chitosan in acetic acid to form asolution. In some embodiments, a peroxide, an azo compound,azobisisobutyronitrile (AIBN), ammonium persulfate, sodium persulfatepotassium persulfate, hydrogen peroxide or tert-butylhydroperoxide maybe added as an initiator. The method further includes heating thesolution to an elevated temperature for a period of time to formuncrosslinked graft copolymer and filtering the solution to obtain acrude product including the uncrosslinked graft copolymer and unreactedchitosan. The unreacted chitosan is removed from the crude product bySoxhlet extraction with ethanol to obtain the uncrosslinked graftcopolymer. In one embodiment, the sodium bisulfite has a concentrationof 0.01-0.03 M in the solution; the potassium persulphate has aconcentration of 0.01-0.03 M in the solution; the chitosan has aconcentration around 0.1 M in the solution; the itaconic acid has aconcentration of 0.1-0.2 M in the solution. The elevated temperature isin the range of 30-60° C. and the period of time is between 1 hour and 6hours. In some embodiments, the concentration of sodium bisulfite,potassium persulphate, the chitosan, and the itaconic acid is 0.01 M,0.01 M, 0.1 M, and 0.1 M respectively in the solution. In someembodiments, the concentration of sodium bisulfite, potassiumpersulphate, the chitosan, and the itaconic acid is 0.03 M, 0.03 M, 0.1M, and 0.2 M respectively in the solution. In some embodiments, thetemperature is 30° C., 35° C., 40° C., 45° C., 50° C., 55° C., or 60° C.and the period of time is about 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, or 6 hours.The Soxhet extraction implemented for the method may be automatedSoxhlet extraction, focused microwave-assisted Soxhlet extraction,ultrasound-assisted Soxhlet extraction, high-pressure Soxhletextraction, fluidized bed extraction or a combination of all suchmethods with similar underlying principles of extraction. In an example,the Soxhet extraction may be combined with modified extraction methodssuch as Soxtec, Soxtherm, or a hot Soxhlet method.

In another exemplary embodiment, the method includes forming thenanocomposite by solution casting a mixture of the uncrosslinked graftcopolymer and aluminum oxide nanoparticles. A homogeneous mixture or asuspension of the uncrosslinked graft copolymer and aluminum oxidenanoparticles is formed and cast to form the nanocomposite. In anexample, the method further comprises dissolving the uncrosslinked graftcopolymer in acetic acid to obtain a polymer solution and adjusting thepH of the polymer solution to the range of 6-7. In some embodiments, theuncrosslinked graft copolymer may be dissolved in a mixture of solventsincluding acetic acid to enhance the rate of dissolution. In an example,the mixture of solvent is a solvent/non-solvent binary mixture.Furthermore, a suspension of aluminum oxide nanoparticles is added inwater portion-wise to the polymer solution to form the mixture andstirring the mixture. The method includes casting the mixture onto acarrier substrate and drying the cast mixture to form the nanocompositefilm. In an example, the aluminum oxide nanoparticles have an averageparticle size of less than 50 nm. In some examples, the average particlesize of the aluminum oxide nanoparticles is less than or equal to 10 nm,15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, or 45 nm.

In an exemplary embodiment, the treating of water includes immersing thenanocomposite in the water and shaking the water at an elevatedtemperature. In an example, the method further includes shaking thewater between 30° C. and 60° C. at a speed of 100-300 rpm for 1-3 hours.In some embodiments, the shaking of the water is done at 30, 35, 40, 45,50, 55, or 60° C. at a speed of 100, 120, 150, 170, 200, 220, 250, 300rpm. In some examples, the water treatment includes use ofnanocomposites in the form of sheets or particles, or films included asa part of water treatment apparatus to filter the water.

In some embodiments, the method includes removing the metal ionscomprising a group including copper, zinc, mercury, cadmium, chromium,lead, gold, uranium, arsenic, sodium, and potassium and alloy or oxideor a mixture thereof. In an example, the method includes removing metalions including at least one of Cu²⁺ and Zn²⁺ from the water. In anotherexample, the metal ions include at least one of Na⁺ and K⁺.

In an exemplary embodiment, the uncrosslinked graft copolymer includesthe side chains of poly(itaconic acid) grafted to the chitosan backbonevia C6 hydroxyl groups. In another example, a grafting density ofitaconic acid is between 20 wt.% and 60 wt.%, the grafting densityincluding poly(itaconic acid) grafted to the chitosan backbone and theitaconic acid acetylating the plurality of amino groups of the chitosanbackbone. In one example, the grafting density of itaconic acid is 20,25, 30, 35, 40, 45, 55, or 60 wt.%.

In one embodiment, the poly(itaconic acid) grafted to the chitosanbackbone has a low crosslink density and exhibits superabsorbentproperties. In one example, the poly(itaconic acid) exhibitsuperabsorbent properties including absorbency rate, absorbency underpressure, and swelling capacity. In an exemplary embodiment, thesuperabsorbent properties of the poly(itaconic acid) grafted to thechitosan backbone are used for wastewater treatment. In anotherembodiment, the superabsorbent properties of the poly(itaconic acid)grafted to the chitosan backbone are used for desalination. In anotherembodiment, poly(itaconic acid) is biodegradable in nature and isobtained via biosynthetic processing of carbohydrates by fungi or otherorganisms.

In various examples, the uncrosslinked graft copolymer includes the sidechains of a poly(itaconic acid) derivative grafted to the chitosanbackbone via C6 hydroxyl groups. In an exemplary embodiment, theuncrosslinked graft copolymer includes the side chains of poly (dialkylitaconate) grafted to the chitosan backbone via C6 hydroxyl groups. Insome examples, the uncrosslinked graft copolymer includes the sidechains of dimethyl itaconate grafted to the chitosan backbone via C6hydroxyl groups. In another example, the uncrosslinked graft copolymerincludes the side chains of di-n-butyl itaconate or diethyl itaconategrafted to the chitosan backbone via C6 hydroxyl groups. In someexemplary embodiments, the uncrosslinked graft copolymer includes theside chains of a poly(dicyclohexyl itaconate), or poly(di-sec-alkylitaconate), or an intermediate such as poly(itaconic anhydride).

In an exemplary embodiment, the uncrosslinked graft copolymer includesthe side chains of poly (dialkyl itaconate) grafted to the chitosanbackbone wherein the side chain length increases with the increase intime during copolymerization. In one example, the maximum percentage ofthe graft increase is between 10 to 20% at 2 to 3 hours’ time during thecopolymerization. In one embodiment, the graft process occurs onto 2grams chitosan at a monomer concentration of 0.15 M and 0.02 Mconcentration sodium bisulfite and potassium persulfate at 40° C. for 5hours.

In the present disclosure, the advantages of the method include that thegraft copolymerization improves the chelating efficiency of the chitosanby constructing extra absorption sites onto the chitosan polymericmatrix suitable for water treatment applications such as desalinationand the removal of metal ions from wastewater. Itaconic acid (IA) isused as a comonomer for the graft copolymerization with the chitosan.The compound exhibits two carboxylic groups enabling the complexation aswell as an ethylenic bond enabling the graft copolymerization reactions.To enhance the water flow through the graft copolymer, nano aluminumoxide is added to it, as dispersed matrix to form the nanocompositefilm.

Yet, another advantage of the present method is the development ofuncrosslinked graft copolymers such that the water desalination, orwater treatment, or wastewater treatment is efficient and enhanced.

In an exemplary embodiment, the nanocomposite is disposed on and/or in aporous membrane in a pipe portion of a device. The membrane extends in alateral plane that is perpendicular to a longitudinal direction of thepipe portion. The membrane has a source water side facing an inlet ofthe pipe portion and a permeate side facing an outlet of the pipeportion. Water passes through the membrane from the source water side tothe permeate side, and metal ions present in the water are adsorbed bythe nanocomposite on or in the membrane. As a result, metal ionconcentration is lower on the permeate side than on the source waterside. The water may be contacted with the membrane at ambient pressure,reduced pressure or elevated pressure. Preferably the water is forcedthrough the membrane at a pressure of at least 1 atm, preferably 1-100atm, 5-50 atm or about 10 atm. For example, the permeate side can beconfigured to be exposed to ambient pressure or reduced pressure whilethe source water side can be configured to be exposed to elevatedpressure. Pressure gradient between the permeate side and the sourcewater side can be adjusted to at least 1 atm, preferably 1-100 atm, 5-50atm or about 10 atm to force the water through the member. In someembodiments, the pipe portion may have a plurality of membranes that arespaced apart so that water passes through the plurality of membranes insuccession. The plurality of membranes may include the nanocompositeand/or other suitable materials for water purification. In someembodiments, the device may have a plurality of pipe portions. Thenanocomposite may be used in conjunction with other membranes. Inanother embodiment, the nanocomposite is immobilized as a film on asupport. The film with the support may be immersed in water in thepresence of ultrasound to adsorb metal ions. In another embodiment, thenanocomposite is processed into particles for filling a cartridgethrough which the water passes.

EXAMPLES Materials

Chitosan (CS) (medium molecular weight) was obtained from Santa Cruzbiotechnology (SCBT). Sodium bisulfite (SBS) was obtained from Acrossorganics. Potassium persulphate (PPS) was obtained from Winlab Limited.Itaconic acid (2-methylenesuccinic acid, 1-propene-2-3-dicarboxylicacid) was obtained from Riedel-de-Häen. Sodium acetate anhydrous, Zincsulphate, Cupric sulfate and 1,4-Dioxan were obtained from BDH ChemicalsLtd., Poole England. Ethanol was purchased from Fisher Chemical. Aceticacid was obtained from Honeywell Fluka™. Potassium chloride and Nitricacid were purchased from Sigma-Aldrich. Nano aluminum oxide waspurchased from Sigma Aldrich. All aqueous solutions were prepared usingdeionized water.

Synthesis of Chitosan Graft Itaconic Acid

In one example, the novel graft copolymerization occurred in two neckedround bottom flasks. An appropriate concentration of initiators (0.01 M,0.02 M and 0.03 M) of Sodium bisulfite (SB) and potassium persulphate(PPS) was added to (2 g) CS in 0.1 M acetic acid., at a specifictemperature (30° C., 40° C., 50° C., 60° C.) in ultrasonic bath of power300 watt. A specific concentration of itaconic acid (IA) (0.1 M, 0.15 M,and 0.2 M) was added, for various intervals of time (1 hour, 2 hours, 3hours, 4 hours, 5 hours and 6 hours). The copolymer was filtered, washedwith ethanol in the Soxhlet extraction to remove the homopolymer. Thecopolymer was dried and weighted, then the graft percentage wascalculated [Int. J. Biol. Macromol., 2014, 68, 21. incorporated byreference herein in its entirety.].

Preparation of (chitosan-g-IA)-Al₂O₃ Nanocomposite

In one example, 1 g of (chitosan-g-IA), was dissolved in 50 mL of 2%(v/v) acetic acid solution on a magnetic stirrer for 10 h at roomtemperature to afford a 2% (w/v) CS solution. The pH of the resulting CSsolution was adjusted to the range of 6-7 by adding the appropriateamount of 1 M NaOH solution under stirring. Now, a suspension of 0.5 gof Al₂O₃ (nano powder, <50 nm particle size Transmission electronmicroscope (TEM), 544833 Sigma-Aldrich) in small amount ofdouble-distilled water was added portion-wise to the (chitosan-g-IA)solution under continuous stirring. The mixture was further stirred for3 h at room temperature, then cast into a 100 mm Petri dish, and driedovernight at 70° C. to remove any acetic acid traces. Finally, theobtained (chitosan-g-IA)/Al₂O₃ nanocomposite film was detached, washedwith distilled water, and dried at 60° C. to ensure that all the solventwas removed completely from the film.

Preparation of Stock Solutions of Metal Ions

In one example, the standard stock solution of Cu (II) or Zn (II) (200ppm) was diluted with deionized water to obtain the desiredconcentrations. The commonly used concentration for stock coppersolution was 10 ppm [Miner. Eng., 2018, 123, 1 incorporated by referenceherein in its entirety.].

Method of Removal of Metal Ions

In one example, 300 mg of graft copolymer nanocomposite were immersed in50 ml of (buffer solution of a definite pH (6, 6.5, and 7) value and anaqueous solution of Cu (II) or Zn (II) ions (10 mg/L)). The content wasallowed to shake, in a shaker, at a speed of 200 rpm, at a giventemperature (30° C., 45° C. or 60° C.), for various intervals of time(1h, 2h, and 3h). The mixture was separated by filtration. Theadsorption uptake was determined by difference in concentration (beforeand after the adsorption process) using Inductively Coupled Plasma (ICP)for quantitative determination. While the residual metal content intothe polymeric matrix was determined as qualitative investigation usingX-ray powder diffraction (XRD) and Scanning electron microscopy andenergy dispersive X-ray spectroscopy (SEM/EDS) [Int. J. Biol. Macromol.,2017, 104, 1495 incorporated by reference herein in its entirety.].

Desalination Process

In one example, the desalination process was conducted using 0.3 g ofCS-g-copolymers into 100 ml of the stock solutions and subjected toshaker for a particular interval of time (1 hour, 2 hours and 3 hours)at 200 revs. min-¹. The suspension was passed through filter-paper andthe residue was analyzed using SEM/EDS.

Results and Discussion

Graft copolymerization of Chitosan (CS) with Itaconic acid (IA), usingPotassium persulphate (PPS) and Sodium bisulfite (SB) as redoxinitiators, was performed, aiming to construct extra adsorption sitesonto chitosan backbone for water treatment applications, such as removalof metal cations from wastewater and desalination of saline water. Todetermine the desired conditions for the achievement of maximum graftpercentage, the main parameters affecting the graft copolymerizationwere studied.

Study of Various Parameters Affecting the Graft Copolymerization

The following equation was used to calculate the grafting percentage.

G%=W − W₀/W₀ × 100%

Wherein G% is the grafting density, W and W₀ represent the weight aftergrafting and weight before grafting, respectively.

Effect of Time

Referring to FIG. 1 , in some embodiments, the effect of variousintervals of time on the grafting copolymerization of IA onto CS isshown. The results show that the percentage of graft increased graduallywith the increase in time due to the increase in the chain length of thegrafting branches until it reached a maximum value. Subsequently, theincrease in the reaction time led to a steady state for the G%, due tothe consumption of the monomer units forming the polymer branches. Themaximum percentage of graft was found to be 19.95% at 5 hours.

Effect of Temperature

Referring to FIG. 2 , in some embodiments, to investigate the effect ofreaction temperature on the graft copolymerization reaction, thetemperature was increased from room temperature to 60° C., allparameters were kept constant. The maximum percentage of graft wasachieved at 40° C. Subsequently, a gradual decrease in percentage ofgraft was observed with the increase in the temperature, which isattributed to the achievement of ceiling temperature of the constructedpolymeric branches.

Effect of Monomer Concentration

Referring to FIG. 3 , in some embodiments, the effect of IAconcentration on the percentage of graft copolymerization is shown (allother parameters were kept constant). The increase of the monomerconcentration increases the graft percentage until it reached a maximumvalue at a monomer concentration (0.15 mol/L). Increasing the IAconcentration showed negligible effect on the percentage of graft, thisis due to the fact that IA does not easily form homopolymers[Materials., 2020, 13, 2707 incorporated by reference herein in itsentirety.]

Effect of Initiator Concentration

Referring to FIG. 4 , in some embodiments, the effect of initiatorconcentration on the graft copolymerization onto CS is shown. A gradualincrease in G% was observed with the increase in initiator concentrationfrom 0.01 M to 0.02 M. Subsequently, a decrease in the G% was observedby increasing initiator concentration, due to the increase in theprobability of chain transfer to initiator reactions. The desiredinitiator concentration was recorded 0.02 M (52.65%). Thus, theappropriate conditions for the graft process onto 2 grams chitosan werefound to be, monomer concentration 0.15 M and 0.02 M initiatorconcentration, at 40° C. for 5 hours.

Characterization of Chitosan Graft IA Copolymer

Referring to FIG. 5 , in some embodiments, to confirm the reaction of CSand IA as well as to determine the centers of reaction between thepolymer and the comonomer, ¹³C NMR spectrophotometer was used. ¹³C-NMRof copolymerization of IA onto CS (G% = 52.65%) is shown in Table 1 andFIG. 5 , the spectral data of parent chitosan is given for comparison inFIG. 6 . The spectral data of Chitosan graft Itaconic acid copolymer(CS-g-IA), as compared to that of chitosan showed the following:

TABLE 1 Structure Carbon atoms ¹³C-NMR(δ ppm)

C1 100 C2 55 C3 70 C4,C4′ 82 C5 79 C6 60 C1′ 95 C2′ 59 C3′ 73 C5′ 75 C6′74 C7 172 C8 38 C9 130 C10 122 C11 174 C12 77 C13 39 C14 45 C15 177 C16179 C17 173 C18 23

I. Broad peaks (from 170 ppm to 180 ppm) correspond to four types ofcarbonyl groups, among which δ = 172 ppm, 174 ppm, 177 ppm, and 179 ppm,correspond to C7, C11, C15, and C16, respectively. In addition to thatalready present in the chitosan structure at σ = 173 ppm, the five peaksare overlapped to form broad peak stating from σ = 172 ppm to 179 ppm.

II. Some of the ethylenic carbons at 129-136 ppm remained unchangedwhile the saturated ethylic ones appear at 39 and 77 ppm.

All the above data confirmed that IA reacted with CS via both graft andcondensation reactions, further confirming utility of performing onlypolyaddition of IA rather than polycondensation occurred ontocrosslinked chitosan backbone [Cao, M. Master Thesis, University of NewHampshire: Durham, 2008 incorporated by reference herein in itsentirety.].

Referring to FIG. 7 , in some embodiments, the morphology of CS-g-IA wasexamined using SEM analysis for further confirmation of the graftingprocess.

Thermal Properties of Synthesized Chitosan Graft Copolymers

To adjust the synthesized copolymers for water treatment applications,their thermal properties was also studied.

Thermogravimetric Analysis (TGA)

The TGA curve is used to determine the initial decomposition temperature(T₀), temperature at which the polymer starts to lose part of itspolymeric matrix. In addition to T₀, the TGA curve shows the percentageof weight loss with the increase in temperature. The value of T₀ dependson the average molecular weight of the polymer [Saudi Pharm. J., 2012,20(3), 263 incorporated by reference herein in its entirety.].

TGA of Chitosan

The CS sample of MW = 20000 (avg) showed a (T₀ = 235° C.). The polymerlost 55% of its weight at 500° C. The TGA curve of CS also showed watermolecules absorption, and while evaporated giving rise to 5% decrease inmolecular weight at 70° C. The graft copolymerization of CS wouldincrease the organic nature of the copolymer matrix by the constructionof the comonomer branches and decrease the water retention of thepolymer.

TGA of Chitosan Graft-Itaconic Acid Copolymers

The grafting process decreases the crystallinity of the CS due to thebranches built onto the main chains, which leads to a decrease in theT₀value. Increasing percentage of the graft means increasing the lengthor the number of branches affecting the secondary forces between thechains and thus, decrease the crystallinity of the polymeric matrix.

Chitosan Graft Itaconic Acid Copolymer

Referring to FIG. 8 , in some embodiments, the T₀ of CS was (235° C.),while the graft copolymers showed a lower value for T₀ as compared toparent CS. The value of T₀ decreased with the increase in the percentageof the graft (also shown in Table 2). However, at 300° C. the weightloss of the highest percentage of the graft (G% = 52.65%) was only 28%as compared to CS 45%. This is due to the extra thermal stability givento the polymeric matrix by the new constructed comonomers. Thus, the IAcomonomer supported the CS backbone at high temperatures.

TABLE 2 Polymer used T₀ Weight loss at 300° C. Weight loss at 500° C.Chitosan 235° C. 45% 55% CS-g-IA (G% =34.35%) 120° C. 37% 45% CS-g-IA(G% = 41.60%) 150° C. 33% 42% CS-g-IA (G% = 52.65%) 165° C. 28% 36%

Applications of Chitosan-g-Itaconic Acid

To enhance the water permeability through the graft copolymers, thealuminum oxide nanocomposite was prepared by the aforementioned solutioncasting method.

Adsorption of Metal Ions From Contaminated Water

The removal of Cu (II) or Zn (II) ions from contaminated water wasinvestigated according to the method described in the experimental part.

The Adsorption Capacity

The adsorption capacity can be calculated using the following equation[N. J. Chem. Soc. Pak., 2019, 41(2), 240 incorporated by referenceherein in its entirety.]

q=(C₀−)/m

C₀, C_(f) (mg/L) are initial and final concentration of metal ion insolution, respectively.

V (L) = Volume of the solution.

m (g) = dry weight of the copolymer.

Effect of pH

The adsorption process is mostly affected by the pH value, which isknown to affect the surface charge of the adsorbent, degree ofionization and speciation of the adsorbate [Carbohydr. Polym., 2016,151, 1091 incorporated by reference herein in its entirety.]. The studyof the influence of pH medium on the cation’s adsorption of CS graftcopolymers as compared to parent CS involved the pH values 6, 6.5 and 7.At acidic pH values, almost 6 or 6.5, deprotonation of functional groups(carboxylic groups) is favored and thus, enhances the electrostaticinteraction between them and Cu (II) ions [Colloid Polym. Sci., 2017,295(4), 627 incorporated by reference herein in its entirety.]. The pHlower than 6 led to the dissolution of CS. Alkaline pH (≥ 7.4) is alsoexcluded from the study as this pH facilitates the precipitation ofCu(OH)₂ [Colloid Polym. Sci., 2017, 295(4), 627 incorporated byreference herein in its entirety.]. Thus, the study of the pH rangeoccurred at pH = 6, 6.5 and 7. It should be understood that the graftcopolymer may be used for pH > 7 in other examples. Referring to theexample of FIG. 9 , the results show that the maximum removal of Cu (II)ions occurred at 6.5, irrespective of the comonomer used. However, incase of Zn (II) ions the maximum removal was obtained at pH = 6, due tothe nature of the metal ions and in aqueous solution as shown in FIG. 10. The following equation was applied.

q = (C₀ − C_(f))50/300 = (C₀ − C_(f)) 1/6

Keeping value of solution = 50 ml and weight of the copolymer = 300 mg,the above equation could be reduced to

q = (C₀ − C_(f))

The difference in solution concentration was determined by ICP.

The results showed that all graft copolymers exhibited higher adsorptioncapacity than the parent CS, irrespective of the cations removed or thepH value. This is due to the extra adsorption sites afforded by thecomonomer’s moieties. Additionally, the adsorption efficiency of CS andits copolymers is higher for Cu (II) ions than for Zn (II) ions.

Effect of Temperature

Referring to the examples of FIGS. 11A and 11B, the effect oftemperature on the adsorption efficiency of CS graft copolymers towardsCu (II) and Zn (II) ions was also studied. The results showed that CSgraft copolymers exhibit slight increase in the adsorption capacity withthe increase in temperature. The results showed that the CS exhibitedconsiderable removal capacity for Cu (II) ions (q = 6.03). The removalefficiency is due to the amino and hydroxyl groups present in the CSstructure, which are considered as adsorbent sites [Carbohydr. Polym.,2016, 151, 1091 incorporated by reference herein in its entirety.]. Thegraft copolymerization of CS with IA afforded the chitosan withcarboxylic groups as repeating units, forming branches as well as othersformed by condensation reactions with some amino groups. These groupsgave the chitosan the extra adsorption sites towards metal cations. Theeffect of the contact time of the copolymer with the aqueous solutioncontaining cations was also investigated.

Effect of the Contact Time

The influence of the contact time on the adsorption process of Cu (II)and Zn (II) ions by CS graft IA copolymer is listed in Table 3. Theresults show that increasing the contact time increased the adsorptioncapacity till time = 2 h. Further increase in the contact time resultedin negligible increase in the cations’ adsorption. Thus, the 2 hours’contact time was chosen as desired contact time for the adsorptionprocess.

TABLE 3 Polymer used q (mg/g) at 1 hour q (mg/g) at 2 hour q (mg/g) at 3hour Cu²⁺ (pH = 6.5, at 60° C.) CS 4.914 5.43 5.58 CS-g-IA (G% = 52.65)9.05 9.77 9.84 Zn²⁺ (pH = 6.5, at 60° C.) CS 3.11 3.43 3.5 CS-g-IA (G% =52.65) 4.5 4.89 4.92

The desired parameters for the removal of Cu²⁺ process was confirmed forpH = 6.5, temperature at 60° C. for 2 hours. However, in the case ofZn²⁺ the maximum removal was obtained at pH = 6, 60° C. for 2 hours.

X-Ray Diffraction

To confirm the adsorption of Cu cations onto the copolymer matrix, theXRD pattern of the copolymers after removal of Cu (II) ions was studied.The copper sulphate crystals exhibited various peaks in its XRD pattern,the most distinguished peaks appeared at 2Θ = 14 and 16, 22 and 35[Colloid Polym. Sci., 2017, 295(4), 627 incorporated by reference hereinin its entirety.]. The XRD patterns of CS-g-IA copolymer, before andafter removal of Cu cations are shown in FIG. 12 .

Referring to the example of FIG. 12 the chitosan-g-IA copolymer is shownbefore (a) and after (b) the removal of Cu (II) ions. The appearance ofan intense peak at 12-14 and disappearance of peaks at 2Θ = 22-40confirmed the scavenging power of the copolymer towards the Cu cations.

Referring to the example of FIG. 13A, the presence of Cu (ions) afterthe removal process by the synthesized copolymer is demonstrated. The Cu(II) ions inside the CS-g-(IA) copolymer matrix are shown in a differentcolor (grey) from the background (black).

Regeneration of Cu (II) Cations

A determination of the recovery percentage occurred using ICP bymeasuring the concentration of metal ion in aqueous solution, after theleaching process. The percentage of recovery is calculated according tothe following equation.

C/C₀ × 100 = Recovery(%)

C = Concentration C of Cu ions after desorption process in aqueoussolution, for a given

C₀ = Concentration of Cu ions for the sample before desorption process.

Referring to the example of FIG. 14 , the adsorption of metal ions ontoa material or desorption of metal ions from a material is highlysensitive to pH of the medium. Thus, the recovery percentage as afunction of the volume of the nitric acid solution was studied. Thequantitative recovery (>95%) was obtained in the entire range of theexperimental frame as shown in FIG. 14 . The maximum recovery wasobtained for 5 ml of HNO₃ (99.5%). The EDS confirmed the leaching of Cu(II) ions out of the polymeric matrix as shown in FIG. 13B.

Reusability of the Graft Copolymer as Metal Removal Material

The synthetized copolymer (G% = 52.65) was reused four times, aftercomplete desorption and washing with deionized water, the results showedthat it can be reused as such without significant loss in its removalefficiency shown in Table 4.

TABLE 4 State of the nanocompo site film Fresh film Recycled film (1)(2) (3) (4) Efficiency (%) 95 94 93 92 92

Desalination

Since the suggested synthesized copolymer was found to act as ionscavenger, desalinizing efficiency towards saline water wasinvestigated. To examine the ability of the graft copolymer to removesalts such as KC1, NaCl from aqueous saline water, the method describedin the experimental method above was used.

The results revealed that the CS-g-(IA) exhibited higher removalefficiency for both KC1 and NaCl than parent CS did. A proposedmechanism representing the scavenging of NaC1 by the CS-g-(IA-DAA)copolymer into its polymeric matrix is illustrated in FIG. 15 . Theproposed mechanism indicates the ability of the graft copolymer toremove both Na and C1 ions from the saline water. Confirmation of theremoval process was observed from the morphology of the graft copolymerafter removal of NaCl as shown in the examples of FIG. 16A and FIG. 16Bby characterizing the Na cations and the Cl anions respectively usingscanning electron microscopy (SEM) with energy-dispersive X-ray analysis(EDX).

Obviously, numerous modifications and variations of the presentdisclosure are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A method of removing a metal ion from water, comprising: treating thewater with a nanocomposite to absorb the metal ion with thenanocomposite and form a polymer-metal ion composite, wherein thenanocomposite is in the form of particles comprising aluminum oxidedispersed in a matrix of an uncrosslinked graft copolymer that includesa chitosan backbone and side chains of poly(itaconic acid) grafted tothe chitosan backbone, the chitosan backbone having a plurality of aminogroups that are acetylated by itaconic acid; and removing thepolymer-metal ion composite from the water.
 2. The method of claim 1,further comprising acidifying the polymer-metal ion composite to removethe metal ion from the polymer-metal ion composite and regenerate thenanocomposite.
 3. The method of claim 2, further comprising treating thewater with the nanocomposite formed by the acidifying to remove themetal ion from the water.
 4. The method of claim 2, wherein the metalion is Cu²⁺ ion and the acidifying includes mixing the polymer-metal ioncomposite with nitric acid to remove the Cu²⁺ ion from the polymer-metalion composite.
 5. The method of claim 1, further comprising forming theuncrosslinked graft copolymer by: adding sodium bisulfite, potassiumpersulphate and itaconic acid to chitosan in acetic acid to form asolution; heating the solution to an elevated temperature for a periodof time to form uncrosslinked graft copolymer; filtering the solution toobtain a crude product comprising the uncrosslinked graft copolymer andunreacted chitosan; and removing the unreacted chitosan from the crudeproduct by Soxhlet extraction with ethanol to obtain the uncrosslinkedgraft copolymer.
 6. The method of claim 5, wherein: the sodium bisulfitehas a concentration of 0.01-0.03 M in the solution; the potassitiumpersulphate has a concentration of 0.01-0.03 M in the solution; thechitosan has a concentration around 0.1 M in the solution; the itaconicacid has a concentration of 0.1-0.2 M in the solution; the elevatedtemperature is in the range of 30-60° C.; and the period of time isbetween 1 hour and 6 hours.
 7. The method of claim 1, furthercomprising: forming the nanocomposite by solution casting a mixture ofthe uncrosslinked graft copolymer and aluminum oxide nanoparticles. 8.The method of claim 7, further comprising: dissolving the uncrosslinkedgraft copolymer in acetic acid to obtain a polymer solution; adjustingthe pH of the polymer solution to the range of 6-7; adding a suspensionof aluminum oxide nanoparticles in water portion-wise to the polymersolution to form the mixture; stirring the mixture; casting the mixtureonto a carrier substrate; and drying the cast mixture to form thenanocomposite film.
 9. The method of claim 8, wherein the aluminum oxidenanoparticles have an average particle size of less than 50 nm.
 10. Themethod of claim 1, wherein the treating comprises immersing thenanocomposite in the water and shaking the water at an elevatedtemperature.
 11. The method of claim 10, further comprising: shaking thewater between 30° C. and 60° C. at a speed of 100-300 rpm for 1-3 hours.12. The method of claim 1, wherein the metal ion includes at least oneof Cu²⁺ and Zn²⁺.
 13. The method of claim 1, wherein the metal ionincludes at least one of Na⁺ and K⁺.
 14. The method of claim 1, whereinthe side chains of poly(itaconic acid) are grafted to the chitosanbackbone via C6 hydroxyl groups.
 15. The method of claim 1, wherein agrafting density of itaconic acid is between 20 wt.% and 60 wt.%, thegrafting density including poly(itaconic acid) grafted to the chitosanbackbone and the itaconic acid acetylating the plurality of amino groupsof the chitosan backbone.